The body size of patients is considered to have an impact on the quality of bowel preparation. The aim of this study was to determine the effect of body mass index (BMI) on bowel preparation and prediction of unprepared patients who underwent colonoscopies. A retrospective study was undertaken with data retrieved from health records at the Center for Gastroenterology and Hepatology in Sulaymaniyah City of 12,527 colonoscopies carried out between February 2012 and December 2018. From the 12,527 records, a total of 9,659 colonoscopy examinations were included in this study. The results showed 21.3% unacceptable colon preparations: 15% poor and 6.3% inadequate. Patients with BMI of 25 and greater accounted for 36.1%. No significant association was found between increased BMI with inadequate preparation (odds ratio [OR]: 1.104, 95% confidence interval [CI]: 0.869-1.401, p value .418 and OR: 0.988, 95% CI: 0.813-1.201, p value .903). However, inadequate preparation could be associated with underweight females, who report constipation, and the elderly. Although there has been shown to be in practice an adverse bias toward patients with a high BMI, this study has shown that an increased body size does not interfere with the quality of bowel preparation or resultant colonoscopy.Copyright © 2021 by the Society of Gastroenterology Nurses and Associates, Inc.